Background
The authors report that "isoprene is produced and emitted by many higher plants and its global emission rate is estimated to be of the order of 500 Tg of carbon (C) per year (Guenther et al., 1995)," noting that "it is very reactive in the atmosphere, acting both to remove hydroxyl radicals (and so prolonging the life time and concentration of methane and other radiatively active gases in the atmosphere) and, under certain conditions, forming ozone, another radiatively active and phytotoxic gas (Monson, 2002)."  Hence, they wanted to determine whether atmospheric CO2 enrichment would either exacerbate or alleviate these deleterious phenomena by either increasing or decreasing, respectively, isoprene emissions from plants, in this case, from English oak (Quercus robur) saplings.

What was done
Possell et al. grew seedlings, one to a mesocosm (16 cm diameter, 60 cm deep), in either fertilized or unfertilized soil in solardomes maintained at atmospheric CO2 concentrations of ambient or ambient plus 300 ppm for one full year, at the conclusion of which period they measured rates of isoprene emissions from the trees' foliage together with their rates of photosynthesis.

What was learned
In the unfertilized trees, the 300-ppm increase in the air's CO2 concentration reduced isoprene emissions by 63% on a leaf area basis and 64% on a biomass basis, while in the fertilized trees the extra CO2 reduced isoprene emissions by 70% on a leaf area basis and 74% on a biomass basis.  In addition, the extra CO2 boosted leaf photosynthesis rates by 17% in the unfertilized trees and 13% in the fertilized trees.

What it means
At one and the same time, the CO2-enriched air of this experiment did a number of positive things.  It (1) enhanced the photosynthetic rates of the trees' leaves, while it (2) reduced their rates of isoprene emission, which tends to (3) reduce the (a) lifetime and (b) atmospheric concentration of (i) methane and (ii) other radiatively active trace gases of the atmosphere, as well as (4) reduce the atmospheric concentration of radiatively-active and phytotoxic ozone.

Not bad for a gas that climate alarmists are trying to characterize as a pollutant.

References
Guenther, A., Hewitt, C.N., Erickson, D. et al.  1995.  A global-model of natural volatile organic-compound emissions.  Journal of Geophysical Research 100: 8873-8892.

Monson, R.K.  2002.  Volatile organic compound emissions from terrestrial ecosystems: a primary biological control over atmospheric chemistry.  Israel Journal of Chemistry 42: 29-42.